If the previous suggested fixes yield no joy, take a look at the diagonal slot at the bottom rear of the bolt. Once the buttstock is off, lay the rifle on the left side, muzzle pointing to the left and the lever pointing away from you. You will see the top of the slot clearly. If a bit of debris gets lodged at the top of the slot, it will block the bolt going into battery as you have described.

The top pic below (pulled from the net)shows the slot in question.

[Linked Image]